1. Field of the Invention
The present invention relates generally to superconducting magnet apparatuses each equipped with a superconducting coil, and more particularly to protection of a superconducting coil during a quench.
2. Description of the Related Art
Superconducting magnet apparatuses are each equipped with, for example, a superconducting coil, an excitation power supply that supplies current to the superconducting coil, and a persistent current switch that forms a closed circuit for supplying a persistent current. Once a portion of the superconducting coil being energized with the persistent current has suffered a transition into a normal conducting state and developed resistance, the resulting occurrence of joule heat will convert stored magnetic energy into heat energy and increase the temperature of the superconducting coil portion which has transitioned into normal electrical conduction. The periphery of the superconducting coil section which has entered the normal conducting state will also suffer a temperature rise due to heat conduction and make a transition from superconductivity into normal electrical conduction. This transition into normal conduction may eventually extend to the entire superconducting coil in rapid sequence, thus resulting in a so-called quench occurring. When the persistent current is flowing through the superconducting coil and this superconducting coil is holding a large volume of stored magnetic energy, if the large volume of stored magnetic energy is converted into heat energy by the quench, a possible excessive increase in the temperature of the superconducting coil might result in thermal damage to the coil.
Consider a case in which the superconducting coil is a high-temperature superconducting coil constructed of a high-temperature superconductor having a critical temperature exceeding 18 K, such as magnesium diboride (MgB2), iron-based superconductor, or oxide superconductor. The critical temperatures of high-temperature superconductors lie in a region that these superconductors have specific heat capacities at least 10 times as great as those of niobium titanium (NbTi), niobium tin (Nb3Sn), and other low-temperature superconductors having critical temperatures below 18 K. Heat conduction due to a quench causes a delay in the propagation of a normal-conducting region. The quench in a high-temperature superconducting coil, therefore, causes a more significant temperature rise than in low-temperature superconducting coils, since stored magnetic energy is consumed locally.
For this reason, JP-1993-190325-A and other related technical documents propose methods of protecting a superconducting coil. In these methods, a protective resistor that receives a supply of current upon a quench event and consumes stored magnetic energy is provided to suppress the consumption of the stored magnetic energy in the superconducting coil. Since the amount of energy that the protective resistor consumes is proportional to the square of the value of the current flowing through the resistor, applying a higher current to the protective resistor yields a greater suppression effect against the temperature rise due to the quench in the superconducting coil. JP-1991-278504-A and other related technical documents propose methods of supplying a high current to a protective resistor. That is to say, the protective resistor and a persistent current switch are each connected in parallel to and across a superconducting coil so that when a quench occurs, a section of a closed circuit composed of the protective resistor and the persistent current switch, this section not being a closed circuit composed of the protective resistor and the superconducting coil, will be electrically disconnected. By so doing, the current that has been supplied to the persistent current switch can be bypassed and induced into the protective resistor. In addition, when a superconducting magnet apparatus is to be operated on a persistent current, a current lead needs to be disconnected from the internal superconducting circuit of a cryostat for suppressed entry of heat into the cryostat, so a protective resistor cannot be connected to the outside of the cryostat. In this case, therefore, the protective resistor is to be connected to the inside of the cryostat and this connection makes it necessary to provide large enough an installation space inside the cryostat. JP-1986-20303-A and the like, for example, propose methods in which a normal-conducting wire to perform the function of a protective resistor is wound around a superconducting coil in order to save the space required for protective resistor connection.